Posted
by
samzenpuson Monday July 07, 2014 @03:10PM
from the get-over-here dept.

KentuckyFC writes The idea that light waves can push a physical object is far from new. But a much more recent idea is that a laser beam can also pull objects like a tractor beam. Now a team of Australian physicists has used a similar idea to create a tractor beam with water waves that pulls floating objects rather than pushes them. Their technique is to use an elongated block vibrating on the surface of water to create a train of regular plane waves. When the amplitude of these waves is small, they gradually push the surface of the water along, creating a flow that pushes floating objects with it. However, when the amplitude increases, the waves become non-linear and begin to interact with each other in a complex way. This sets up a flow of water on the surface in the opposite direction to the movement of the waves. The result is that floating objects--ping pong balls in the experiment--are pulled towards the vibrating block, like a tractor beam.

If the backwards flow of water is a result of a complex system of causal interactions, they couldn't have come to the conclusion that this would work based on what they already knew. So how did they discover it? Was it an accident? If not, can one of them look into the future? This is a pretty awesome result if it didn't depend on coincidence.

Have you ever taken a bath and noticed that there's a dead spider or something in the tub? You definitely don't want to touch the dead spider, but you do want to transport it down to the far end of the bath. So you try creating some waves. Somehow, the effect of the waves is to bring the spider closer to you, instead of moving it further away. Then, if you're a physicist, you say, "That's odd...."

they couldn't have come to the conclusion that this would work based on what they already knew.

They had some good expectation for something like this to work because similar systems were built using light, it was just a matter of getting similar setups with messier water waves. The paper covers a piece of the thought process and once could see that the principles it is built on could be thought out before hand, although sometimes just building something is easier when dealing with something that might be borderline (e.g. exactly what size vorticies are produced and in what quantity, etc., although d

Pushing is nowhere near as effective in cleanup operations which is why vacuum cleaners suck instead of blow (and very early victorian models did). Suction concentrates the particles in a fixed location whereas blowing scatters them.

Tell this to all the gardeners and landscaping crews in the country, as instead of raking up leaves now, their favorite toy seems to be the leaf blower to scatter all the debris to neighboring yards so they can get paid to do it there next.

Sucking has rather short range unless you collect a lot of water, which in turn means high energy needs. I don't know what the energy demands of a wave-sucker would be, or some form of hybrid device, but there may potentially be a saving. It could also be done using fewer moving parts, as the oscillatory motion of a wave-maker is achievable with just a magnet and a coil. You'd want to pump it at the natural resonant frequency for maximum efficiency.

Baby-brain.... how is light always linear? I thought that the slit experiment was evidence of non-linear light? Also, doesn't a photon colliding with an electron potentially release another photon whose trajectory is influenced by the angular momentum of the parent electron?

Does it only happen when I have 3 walls that the waves can bounce off of? Because that would not work in an open water setting, only in enclosed settings. It makes me think of pool (billiards) where you could make a ball come toward you by hitting the que off two walls then into the back of the target ball.

If you can repeat it from arbitrary points and arbitrary distances then you start to have something useful.If you can repeat it with other wave sources then it gets more useful.

FWIW, this paper [arxiv.org] talks about doing this with light (in the context of micro-manipulation). Doesn't look like we will be using this for any star-ship sized objects in the near future...

The basic idea is that you use a light with a specific profile to stimulate the object you want to attract in a way that causes a scattering field such that there is a net force backward to the emitter (it only works if the amount of net forward momentum of the light is relatively small compared to the scattering).

The water stuff referenced by this article works on a completely different principle, though as described here [arxiv.org].

They are similar in that they originate with a wave generator, also hitting the target at a glancing angle is a way to achieve the necessary conditions and both provide a net attractive force (aka tractor beam), but the physics is totally different.

When you say that a relatively large scattered field (as compared to the relative momentum of the light beam used) attracts objects - how does a 'scattered field' cause this attraction? I guess I'm trying to determine how a scattered field of light would cause momentum towards the emitter...